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UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station. 


URBANA,    MAY,  1902. 


BULLETIN  NO.  72. 


ADDITIONAL  INSECTICIDE  EXPERIMENTS  FOR 
THE  SAN  JOSE   SCALE. 


BY  S.  A.  FORBES,  STATE  ENTOMOLOGIST. 


The  last  observations  reported  in  Bulletin  71  of  the  Illinois 
Agricultural  Experiment  Station  were  made  March  25,  at  which 
time  it  now  appears  that  the  insecticide  effect  of  the  experimental 
applications  made  to  trees  infested  by  the  San  Jose  scale  was  not 
yet  complete,  or  at  least  not  yet-fully  manifest.  At  the  above  men- 
tioned date  there  were  found  on  trees  which  had  been  treated  with 
the  Oregon  and  California  washes  twenty  and  twenty-two  days 
previously,  living-  scales  in  numbers  varying  from  six  to  thirty-one 
per  cent,  of  those  alive  in  the  beginning-  for  trees  treated  with  the 
California  wash  and  from  one  to  seventeen  per  cent,  for  those 
treated  with  the  Oregon  wash  (see  tables  in  Bulletin  71). 

A  careful  examination  of  these  experimental  trees,  made  by 
Mr.  E.  S.  G.  Titus  May  12,  and  a  systematic  count  of  dead   and 

265 


266  BULLETIN    NO.    J2. 

living  scales  showed  that  by  that  time  extremely  few  scales  re- 
mained alive  on  any  of  these  trees.  Five  hundred  young  scales  of 
the  preceding-  year  were  critically  examined  on  each  of  eighteen 
trees — nine  thousand  scales  in  all — care  being-  taken  to  choose  lots 
from  all  parts  of  the  tree  up  to  the  terminal  twig-s.  Of  these  nine 
thousand  scales,  only  thirty-five  were  living-,  the  ratio  of  living-  to 
dead  varying-  from  none  at  all  to  a  maximum  of  1  per  cent. — as 
near  complete  destruction  as  any  field  operation  is  likely  to  ac- 
complish. 

One  half  the  trees  on  which  these  counts  were  made  had  been 
sprayed  with  the  California  wash  and  the  other  half  with  the  Ore- 
gon wash  ;  and  sixteen  of  them  were  chosen  in  pairs  such  that  the 
only  difference  between  the  treatment  of  the  trees  of  each  pair  was 
the  difference  in  the  insecticide  applied.  Comparison  shows  that 
at  the  time  these  counts  were  made  all  the  difference  of  effect  be- 
tween the  two  insecticides  had  disappeared,  one  proving  finally  as 
efficient  as  the  other. 

These  eighteen  trees  were  so  selected  as  to  represent  ten  vari- 
ations of  treatment  with  water,  ranging  from  daily  spraying  for 
seven  days  in  succession  beginning  with  the  next  day  after  the  in- 
secticide treatment,  to  single  applications  of  water  after  an  inter- 
val so  long  as  to  have  practically  no  effect.  On  a  comparison  of 
the  reports  concerning  the  different  lots  of  these  trees,  I  find  no 
evidence  that  variations  with  the  water  treatment  made  any  dif- 
ference with  the  final  effect  of  the  insecticide.  The  destruction 
of  the  insects  was  retarded  in  some  cases  by  frequent  and  early 
wettings,  as  shown  in  Bulletin  71,  but  was  practically  complete 
in  every  instance  before  May  12. 

By  those  who  have  read  Bulletin  71  (see  page  243)  it  will  be 
remembered  that  large  percentages  of  the  young  scales  of  the  pre- 
ceding year  were  dead  at  the  beginning  of  the  experiment  in  con- 
sequence apparently  of  the  character  of  the  weather  of  the  preced- 
ing summer.  If  allowance  is  made  for  this  fact  and  the  ratio  of 
living  to  dead  scales  is  reckoned  with  reference  to  those  alive  when 
the  experiment  began,  we  find  that  the  ratio  of  scales  still  living 
on  these  eighteen  trees  on  the  12th  of  May  varies  from  none  at 
all  for  five  of  the  trees  to  2.7  per  cent,  for  one  tree,  the  average 
ratio  of  living  scales  for  the  eighteen  trees  being  a  little  less  than 
.8  per  cent. 

In  preparing  Bulletin  71  I  had  no  data  in  hand  for  a  compari- 
son of  the  insecticide  effects  of  the  lime  and  sulphur  washes  with 
whale-oil  soap  or  petroleum,  but  Mr.  Titus's  visit  of  May  12  en- 
ables me  to  make  good  this  deficiency.  All  the  infested  trees  at 


IQO2.]  ADDITIONAL  INSECTICIDE  EXPERIMENTS  FOR  SAN  JOSE  SCALE.  267 

this  place  not  used  in  the  experiment  had  been  sprayed  with  whale- 
oil  soap  in  March  by  one  of  my  regular  field  parties,  and  an  ex- 
amination of  five  thousand  scales  on  eighteen  of  the  trees  so  treated 
gave  a  total  of  39  scales,  a  little  more  than  twice  the  ratio  of  living 
to  dead  scales  found  on  those  which  had  been  treated  with  the 
lime  and  sulphur  washes. 

From  the  foregoing  it  appears  that  certain  of  the  statements 
made  in  Bulletin  71  must  be  modified  with  reference  to  these  later 
and  more  complete  results.  It  may  be  safely  said  that  the  washes 
applied  were  extremely  efficient  insecticides.  Even  such  failures 
to  kill  the  scales  as  occurred  were  probably  due  to  imperfect  dis- 
tribution of  the  spray.  The  action  of  the  washes  is  more  prolonged 
than  I  have  been  previously  led  to  infer,  extending  evidently  be- 
yond three  weeks,  and  although  at  first  considerable  differences 
were  noticeable  between  the  Oregon  and  California  washes,  we 
must  conclude  from  the  above  report  that  these  are  differences  in 
promptness  and  rapidity  of  action  rather  than  in  the  final  effect. 
A  similar  statement  may  be  made  with  reference  to  the  effects  of 
rain  as  represented  by  the  application  of  water  to  our  experimental 
trees.  This  clearly  has  the  effect  to  delay,  but  not  to  prevent,  a 
complete  destruction  of  the  scale,  and  it  is  apparent  that  these 
western  washes,  costing  $1.12  per  hundred  gallons,  are  at  least  as 
destructive  to  insect  life  as  the  solution  of  whale  oil  soap,  costing 
$6.50  for  the  same  quantity. 

The  following  table  will  give  some  details  of  observation  not 
reported  in  the  text.  The  numbers  used  for  the  trees  are  the 
same  as  those  in  the  tables  of  Bulletin  71,  pages  249-258,  and  the 
experimental  history  of  each  tree  can  also  be  found  in  that  bulle- 
tin. It  is  to  be  understood  that  five  hundred  scales  were  counted 
for  each  tree  May  12.  The  first  tree  of  each  pair  was  sprayed 
with  the  California  wash,  and  the  second  with  the  Oregon  wash. 


268 


BULLETIN  NO.   72. 
TABLE   OF   COMPARATIVE    RESULTS. 


[May,  1902. 


No.  of  Tree. 

Date   of  Treat- 

Per cent,    alive 

Per  cent,  killed 

Per  cent,  killed 

ment. 

when  treated. 

March  25. 

May  12. 

I 

March   3 

40 

79 

IOO 

ii 

3 

67 

93 

98.8 

i 

3 

00 

98.9 

12 

3 

66 

95 

99.4 

3 

3 

29 

78 

98.6 

13 

"         3 

57 

94 

98-9 

6 

3 

64 

86 

IOO 

14 

3 

49 

92 

99.6 

9 

3 

56 

89 

99-6 

16 

"         3 

48 

93 

99.6 

10 

3 

69 

95 

99-4 

28 

5 

58 

98.6 

4 

5 

42 

78 

IOO 

18 

5    , 

47 

88 

98.3 

30 

5 

44 

89 

IOO 

5 

37 

85 

97-3 

36 

5 

97 

IOO 

24 

5 

42 

93 

99 

Averages. 

5° 

89 

99.2 

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